1. Field of the Invention
This invention relates to the recording of program materials such as speech or music and more particularly to devices for preventing the illegal copying of these recorded materials.
Unauthorized copying of magnetic tapes, often termed "piracy" or "pirating", is a widespread activity. As a result of these acts, the revenues of tape publishers or their licensees have been significantly diminished. Accordingly, a need exists to equip original recordings with a means to prevent unauthorized copies from later being made.
2. Description of the Prior Art
Inventions relating to the prevention and detection of unauthorized tape duplications have already been described by British Pat. No. 1,525,292 and U.S. Pat. Nos. 4,040,099 and 4,086,634.
The first names patent refers to a technique for identifying the source from which an unauthorized recording was made. After the original recording is completed, imperceptible amounts of information are erased from the tape (or disk) in a particular pattern. If the tape is later duplicated, the pattern of gaps will be duplicated as well, thereby indicating the source of the copy.
The second named patent exploits the approximately 90.degree. phase shift between low frequency recorded material and high frequency recorded material inescapably introduced by the duplicating hardware now in use. The invention teaches a technique for preparing the original tapes by inserting predetermined high and low frequency signals in particular narrow frequency bands and a method for comparing the phase of these signals with the phase of signals recorded on a suspected tape. Any phase shift present would be readily detected through the use of an oscilloscope.
The third named patent refers to a technique for destroying the audio quality of a duplication by introducing beat or cross-modulation interference signals. It teaches a method for recording a high frequency modulated signal together with the audible program in a manner that will produce on the copy detectable and identifiable interference signals arising between the modulated signal and conventional high frequency recording bias signal.
Heretofore, no techniques have been disclosed that prevent the recording mechanism of the copying tape deck from functioning, thereby not permitting any of the program material from being copied whatsoever.